Question

I am trying to write a nodejs bindings for a C++ library and I seem to have hit a roadblock.

I am working on trying to make all the calls to the C++ library asynchronous and thats why I am using libuv. I am basically following this tutorial.

I want to be able to call class member functions from libuv’s uv_queue_work. Have a look at this code —

class test {
  private:
    int data;
    void Work(uv_work_t *req);
    void After(uv_work_t *req);
  public:
    Handle<Value> Async(const Arguments& args) {
      HandleScope scope;
      Local<Function> callback = Local<Function>::Cast(args[0]);
      int status = uv_queue_work(uv_default_loop(), **something**, Work, After);
      assert(status == 0);
      return Undefined();
    }
};

Basically I expect the Work and After functions to work on the data element of the class. However this doesnt seem to work. I have tried typecasting the pointers to Work and After after from type void test::(*)(uv_work_t*) to void (*)(uv_work_t*). But that also doesnt seem to work.

Could you guys give me some tips on how to work around this??

Answer 1

So as you’ve realized, you cannot call the member functions directly.

The second argument “something” is of type uv_work_t, which has a member “void* data”.

What you will need to do is create static methods inside your class for “Work” and “After”, create a uv_work_t structure, and assign data to “this”.

Once that is done inside your static “Work” and “After” methods you do a static cast on “req->data” (To your class type) and then call your member functions.

For example:

uv_work_t* baton = new uv_work_t();
baton->data = this;
int status = uv_queue_work(uv_default_loop(), baton, StaticWork, StaticAfter);

And then in the static methods

test* myobj = static_cast<test>(req->data);
myobj->Work();

And similar code for the StaticAfter function